Picture coding using adaptive color space transformation

ABSTRACT

The present invention is based on the finding that pictures or a picture stream can be encoded highly efficient when a representation of pictures is chosen that is having different picture blocks, wherein each picture block is carrying picture information for picture areas smaller than the full area of the picture and when the different picture blocks are carrying the picture information either in a first color-space representation or in a second color-space-representation. Since different color-space-representations have individual inherent properties with respect to their describing parameters, choosing an appropriate color-space-representation individually for the picture blocks results in an encoded representation of pictures that is having a better quality at a given size or bit rate.

The present invention relates to picture coding and in particular to aconcept allowing for a more efficient coding of picture content, i.e.producing encoded representations of pictures or picture streams havinga better R/D-ratio.

BACKGROUND OF THE INVENTION

Applications where pictures or picture streams have to be encodedefficiently are numerous. For example, still image compression isnormally done by digital photo cameras to increase the number ofpictures that can be stored on a storage medium of a given size. When itcomes to transmission of image sequences or complete movies over atransmission medium offering only limited bandwidth, the use of anefficient codec (coder-decoder) that allows for a high compression ofthe content of the pictures becomes even more urgent. This is on the onehand due to the desired transmission over transport channels offeringlow bandwidth, such as the streaming of video content to mobile phones.On the other hand, the transmission of high-resolution video content isbecoming more and more popular since displays capable of displaying suchhigh resolution pictures are spreading more and more amongst consumers.One major trend is the upcoming broadcast of high-definition television(HDTV).

In general, two different coding approaches may be distinguished, thefirst aiming for an encoding without any loss of information and thesecond accepting a (moderate) loss of information and quality to achievea significant reduction in file sizes. Although lossless encodingtechniques exist for both still images and movie content, thesetechniques, often based on entropy coding, cannot achieve a file-sizereduction being sufficient or acceptable for the desired application.Therefore, lossy compression is mostly advantageous such as JPEG forstill image compression and MPEG 2 for movie compression.

Generally, lossy compression has the problem of a decreased quality ofthe compressed pictures compared to the underlying original picture.Naturally, the quality of the picture becomes worse when the compressionrate is increased, i.e. when the file size of the compressed picture isdecreased. Therefore, one has to find a compromise between the desiredquality of a compressed image and the file size acceptable fortransmission or storage. Mostly, the decrease in file size and also theloss in information is achieved by quantization of parameters describingthe picture properties and hence, the coarser the quantization the worsethe quality and the smaller the compressed picture. The quality of thecompressed picture is commonly estimated by a comparison of thecompressed picture with the underlying original picture. This allowsestimating a signal-to-noise ratio, wherein the noise is understood tobe the noise introduced during the compression.

In current compression algorithms, a block-wise processing of images iswidely used. The underlying basic idea is that for normal image content,a change of content, e.g. of color and brightness, of neighboring pixelsis normally relatively small. Therefore, by using areas of neighboringpixels that are processed and compressed together, one should achieverather high compression rates without significantly reducing theperceptual quality of the picture. Such a picture block is from here onalso referred to as macro-block. Thus, in other words, the macro-blocksserve as a kind of sub-picture unit in coding. The block-subdivision isillustrated in FIG. 7, where a picture 10 is subdivided into 12 equallysized picture blocks 12A to 12L. The subdivision into 12 differentpicture blocks is to be understood as an example only.

As an example, a single picture block 12 I is magnified in FIG. 7,wherein the subdivision of the picture block 12 I into an 8×8 matrixshows the single pixel elements building the macro-block 12I. Also here,the formation of a picture block from 8×8 individual pixels is to beunderstood as an example only. To represent color within each individualpixel, each pixel is assigned three parameters holding different colorinformation in a certain color space.

One simple approach of encoding a macro-block is to quantize the threeparameters of each single pixel and to perform an entropy coding on thequantized parameters after the quantization. Since quantizationsignificantly reduces the available parameter space for the entropycoding, quantization of the parameters can already reduce the amount ofstorage space or bits needed to describe one macro-block significantly.

However, in order to reduce the amount of syntax elements describing thepicture content having high energy, the picture information within onemacro-block is often described by transformation coefficients, generatedby transforming the picture content within the macro-blocks into anotherrepresentation (spectral domain). One example is to perform a discretecosine transformation, eventually on a sub-macro-block level, and to usethe transformation coefficients as the image information, which may thenbe quantized and which might also be entropy coded after quantization.

The transformation may, for example, be applied to the complete pixelinformation, i.e. three parameter values per pixel of the picture block12I. Advantageously, the transformation is performed separately for thethree parameters/components.

For further reduction of file sizes and higher compression, one may alsomake use of a property of the human eye, which seems to put more weighton brightness information than on color information when judging theperceptual quality of an encoded picture. Therefore, one possibility toenhance the coding performance (with respect to quality and bit rate) isto reduce the number of color parameters with respect to the number ofbrightness parameters within a macro-block. That is, the informationbasis, on which a representation based of transformation coefficients isbased, contains more information on brightness within the picture blockthan on color. Since there are numerous ways to describe a color by onesingle brightness-value and two color-values, the brightness-value shallbe referred to as luma-value and the color-values shall be referred toas chroma-values from here on.

One possible way of building such a picture block 12I, suited to betransformed, is indicated in FIG. 7. The magnified picture block 12I has8×8 individual pixels, each pixel normally described by one luma and twochroma values. FIG. 12I exemplifies a way to reduce the amount ofchroma-information in that only the chroma information of specificpixels is used as the data set underlying the transformation. This isindicated by the letter C within each individual pixel that is part ofthe chroma-data set. On the contrary, the most important lumainformation of every individual pixel is used.

It is to be understood that the situation shown in the magnifiedmacro-block 12I is an example only. It is also possible to furtherreduce the amount of chroma information. This could, for example, beachieved by omitting every second chroma information, that is for everyeight luma values, one chroma value would be taken into account duringthe transformation. It would also be possible to not simply use thechroma-values of the pixels shown in FIG. 12A but to calculate anaverage chroma value from four neighboring pixels by averaging thechroma value of the pixels. Such a chroma value would then be assignedto a position within the macro-block that is lying in the center of thefour underlying pixels, as indicated by chroma value 16 indicated inFIG. 7.

The encoding techniques described above can generally be used for bothstill images and moving pictures. For moving pictures, moresophisticated methods of encoding are used, involving motion estimation.

In case of macro-block-wise motion estimation, two (or more) pictures ofa picture stream (the pictures do not necessarily have to directlyfollow each other) are located which show the same picture content inthe two images. In the simplest case, the picture content within themacro-block of a current frame has not changed compared to the referenceframe. However, the content of the macro-block may appear at a slightlydifferent position in the reference frame. In this case it is sufficientto know the motion vector of the movement of the picture content duringthe transition from the reference picture to the macro-block of thecurrent picture to reconstruct or predict the picture information of thepicture block in the current picture, once the reference picture iscompletely known at the decoder side. Of course, normally there areslight changes within the picture block during the transition from thereference picture to the current picture. Due to this, the predictionerror is also transmitted thereby allowing to reconstruct the change ofpicture content in the macro-block along with the motion vector, toallow for a complete reconstruction of the macro-block in the currentpicture. Codecs which use motion prediction with subsequent residualcoding such as transformation and entropy coding are called hybrid videocodecs.

According to state of the art techniques, predictive coding allows foran efficient representation of picture sequences. In predictive coding,first a value for a quantity to be coded is predicted and then only thedifference of the really observed value to the predicted value is codedand transmitted. This will also yield a gain in bit rate, since having areliable prediction, the difference parameters will on the average besmaller than the absolute parameters describing the picture within themacro-block. Hence, the symbol space on which a subsequent entropycoding (with or without preceding quantization) is based can bedecreased, allowing for shorter code words and such for a reduction inbit-rate.

Although there have been quite some efforts undertaken to decrease thefile size of compressed pictures or movies which are compressed usingblock-wise coding strategies without unacceptably decreasing theperceptual quality of the compressed content, the properties of thesingle picture blocks are still not exploited optimally with respect todifferent parametric representations of picture blocks.

SUMMARY

According to an embodiment, a decoder for reconstructing a picturerepresented in an entropy encoded representation having a first and asecond picture block, the picture blocks carrying picture informationfor picture areas smaller than the area of the picture, wherein thefirst picture block is carrying the picture information in a firstcolor-space representation and the second picture block is carrying thepicture information in a second color-space representation, may have: anentropy decoder for deriving the first and second picture blocks usingan entropy decoding rule; and a color-space transformer for transformingeither the color-space representation of the first picture block to thesecond color-space representation or the color-space representation ofthe second picture block to the first color-space representation.

According to another embodiment,an encoder for generating arepresentation of a picture having a first and a second picture block,the picture blocks carrying picture information for picture areassmaller than the area of the picture in a first color-spacerepresentation, may have: a color-space transformer for derivingtransformed picture blocks, the color-space transformer adapted totransform the picture information of one of the first and the secondpicture blocks to a second color-space representation; and an entropyencoder for deriving an entropy encoded representation of thetransformed picture blocks according to an entropy encoding rule.

According to another embodiment, a method of decoding a picturerepresented in an entropy encoded representation having a first and asecond picture block, the picture blocks carrying picture informationfor picture areas smaller than the area of the picture, wherein thefirst picture block is carrying the picture information in a firstcolor-space representation and the second picture block is carrying thepicture information in a second color-space representation, may have thesteps of: entropy decoding the entropy encoded representation forderiving the first and second picture blocks using an entropy decodingrule; and transformation of either the color-space representation of thefirst picture block to the second color-space representation or thecolor-space representation of the second picture block to the firstcolor-space representation.

According to another embodiment, a method of generating a representationof a picture having a first and a second picture block, the pictureblocks carrying picture information for picture areas smaller than thearea of the picture in a first color-space representation, may have thesteps of: transformation of the picture information of the first or ofthe second picture block to a second color-space representation toderive transformed picture blocks; and entropy encoding the transformedpicture blocks for deriving an entropy encoded representation of thetransformed picture blocks according to an entropy encoding rule.

According to another embodiment, a parameter bit stream may have anentropy encoded representation of a picture having a first picture blockand a second picture block, the picture blocks carrying pictureinformation for picture areas smaller than the area of the picture,wherein the first picture block is carrying the picture information in afirst color-space representation and the second picture block iscarrying the picture information in a second color-space representation.

Another embodiment may have a computer program for performing, whenrunning on a computer, the method of decoding a picture represented inan entropy encoded representation having a first and a second pictureblock, the picture blocks carrying picture information for picture areassmaller than the area of the picture, wherein the first picture block iscarrying the picture information in a first color-space representationand the second picture block is carrying the picture information in asecond color-space representation, wherein the method may have the stepsof: entropy decoding the entropy encoded representation for deriving thefirst and second picture blocks using an entropy decoding rule; andtransformation of either the color-space representation of the firstpicture block to the second color-space representation or thecolor-space representation of the second picture block to the firstcolor-space representation.

Another embodiment may have a computer program for performing, whenrunning on a computer, the method of generating a representation of apicture having a first and a second picture block, the picture blockscarrying picture information for picture areas smaller than the area ofthe picture in a first color-space representation, wherein the methodmay have the steps of: transformation of the picture information of thefirst or of the second picture block to a second color-spacerepresentation to derive transformed picture blocks; and entropyencoding the transformed picture blocks for deriving an entropy encodedrepresentation of the transformed picture blocks according to an entropyencoding rule.

The present invention is based on the finding that pictures or a picturestream can be encoded highly efficient when a representation of picturesis chosen that has different picture blocks, with each picture blockcarrying picture information for picture areas smaller than the fullarea of the picture, and when the different picture blocks carry thepicture information either in a first color-space representation or in asecond color-space-representation. Since differentcolor-space-representations have individual inherent properties withrespect to their describing parameters, choosing an appropriatecolor-space-representation individually for the picture blocks resultsin an encoded representation of pictures that has a better quality at agiven size or bit rate.

In one embodiment of the present invention, an inventive decoder isused, that receives a bit stream having different picture blocks, thepicture blocks carrying picture information either in a firstcolor-space-representation or in a second color-space-representation.The decoder further receives a transformation flag, indicating whetherthe color-space-representation of the picture block presently operatedon is to be transformed into a different color-space-representation ornot. Such a decoder allows for the reconstruction of image blocks withinan image decoding process that are encoded in differentcolor-space-representations. The decoder is therefore operative toprocess an inventive bit stream which allows for a more compactrepresentation of a picture or a picture stream without decreasing thepicture quality.

In a further embodiment of the present invention, an inventive decoderis used which is operative to process picture blocks in aRGB-representation and in a representation, in which the color and thebrightness information is stored by separate parameters, i.e. arepresentation having one luma-parameter and two chroma-parameters. Thisis advantageous in that normally image material is present in theRGB-color-space and can therefore be processed by the inventive decoder.Additionally, inherent differences of the parameter values of differentcolor-space representations can be advantageously made use of to providean optimal reproduction quality at a given bit rate.

In a further embodiment of the present invention, an inventive decoderhas a color-space transformer that is operative to perform thecolor-space-transformation on a parametric representation of the pictureblocks, wherein the parametric representation describes the pictureblock in a transform domain, for example in a frequency domain. This hasthe great advantage that in conventional picture processing, picturedata is normally transformed prior to transmission to allow for anefficient quantization. Therefore, an inventive decoder that isoperative to also work in the transform domain can be easily implementedinto conventional designs to further increase the coding efficiency ofthose designs.

In a further embodiment of the present invention, an inventive decoderis integrated into a picture or video decoder that further has arequantizer and an entropy decoder. Such, the inventive decoder can beused within the picture or video decoder to further increase the codingefficiency in that a video decoder or a picture decoder having aninventive decoder is enabled to process inventive, highly compressed bitstreams.

In a further embodiment of the present invention, an inventive decoderis operative to switch the color-space-transformation on and offdepending on a transformation flag present in a provided bit stream.Such an inventive decoder can therefore be implemented into conventionaldesigns and allows both conventional decoding and decoding inventive bitstreams within one single device.

In a further embodiment of the present invention an inventive encoder ishaving a color-space-transformer for transforming thecolor-space-representation of picture blocks from a “natural”color-space-representation (i.e. the color-space representation in whichthe content is originally created) to a secondarycolor-space-representation when a transformation decider is indicatingthe desired transformation. The transformation decider is operative toestimate, on a block basis, the expected quality of the encoded picturerepresentation when the respective blocks are encoded in the naturalcolor-space-representation or in the secondarycolor-space-representation. The inventive transformation decider istherefore also operative to decide whether a transformation is needed orappropriate for the individual blocks on the basis of a desired maximumbit rate and hence choosing the best possible coding quality at a givenbit rate. This has the great advantage that implementing the inventiveconcept allows for lower bit rates than conventional techniques whilepreserving the same perceptual quality.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be detailed subsequentlyreferring to the appended drawings, in which:

FIG. 1 shows an embodiment of an inventive encoder;

FIG. 2 shows a bit rate versus quality graph of differentcolor-space-representations;

FIG. 3 shows an example for a color-space-transformation emphasizing theinventive concept;

FIG. 4 shows an embodiment of an inventive encoder;

FIG. 4A shows an example of a given context for context based coding;

FIG. 5 shows an example of an encoding concept for an embodiment of aninventive encoder;

FIG. 6 shows an example of an inventive bit stream; and

FIG. 7 shows block-wise decomposition of a picture for subsequentpicture processing.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an inventive decoder 100. The decoder 100 is having acolor-space transformer 102 that is operative to transform a pictureblock from a first color-space-representation (A) to a secondcolor-space-representation (B) and vice versa. The decoder is usedwithin the reconstruction of a picture or a movie that is represented ina representation having a first picture block and a second picture blockwithin pictures, wherein the picture blocks carry their pictureinformation in a first color-space-representation (A) or in a secondcolor-space-representation (B). The decoder 100 receives a bit stream104 comprising several picture blocks 104A to 104D as an input, whereinthe picture blocks 104A to 104D are included within the bit stream 104in different color-space-representations A or B.

The color-space transformer 102 within the decoder 100 receives selectedpicture blocks to convert them from their originalcolor-space-representation to a desired color-space-representation. Ascan be seen by the output bit stream 106 of the decoder 100, in theexample given in FIG. 1, the color-space transformer is operative totransform the color-space-representation (B) of the picture blocks 104Cand 104B to color-space-representation A such that after the decodingall picture blocks within the output stream 106 are represented in thecolor-space-representation A.

In a modification of FIG. 1, the decoder 100 can furthermore comprise aflag receiver 108 for receiving a transformation information transmittedwithin the bit stream that indicates whether a corresponding pictureblock has a color-space-representation that shall be transformed or not.Depending on the received transformation indication, the flag receiver108 can either direct a picture block to the color-space transformer ordirectly to the output of the decoder (100).

Although in an advantageous embodiment an inventive decoder receives atransformation indication signal with the bit stream, it is alsopossible to implement a decoder that recognizes by some recognizationalgorithm, whether a color-space transformation is required or not for acertain picture block. This could, for example, be derived from thepicture block element representation itself.

In a further embodiment of the present invention an inventive decoder isoperative to receive an additional activation flag that is activating ordeactivating the color-space transformer for a number of consecutiveframes (slices) or, more general, for larger groups of picture blocks.

It is a further advantageous embodiment of the present invention toimplement the inventive decoder in a video decoder which is operative toreceive a bit stream signal that is comprising picture information ofpicture blocks that are encoded in a predictive coding scheme based onmotion estimation of picture blocks.

In such a predictive coding scheme, only the difference or residual(difference macro-blocks) between the motion-compensated prediction forthe picture blocks and the actual content of the picture blocks istransmitted to increase the encoding efficiency. In one embodiment ofthe present invention, these differential macro-blocks are transmittedand decoded either in a primary (e.g. RGB) or in a secondary (e.g.YCoCg) color-space-representation. Therefore, the already rather compactinformation describing the differential picture blocks can be furtherdecreased by a simple color transformation, which is computationallycheap. When it comes to coding of differential signals i.e. signals thatare intended to have describing parameters of low values (i.e. smallnumbers), the effect of a color-space transformation may be extremelybeneficial. This will shortly be motivated in the following descriptionof FIGS. 2 and 3.

In the context of residual signals, the present invention describes atechnique for switching between a primary (e.g. RGB) and a secondary(e.g. YCoCg) color-space in order to adapt thecolor-space-representation of the prediction residual signal to thecharacteristics of the given video source and the specific codingconditions at hand. By using the inventive concept and techniques, anencoder may choose between two alternative color representations of theresidual signal for each single macro-block (picture-block) in arate-distortion optimal way. The encoder's choice may be signaled to acorresponding decoder by means of a macro-block-based flag. In anadvantageous embodiment of the present invention, the inventive conceptmay be applied to advanced video codecs such as H.264/MPEG4-AVC and isparticularly useful to reduce the demanded bit rate in high-qualitycoding scenarios of those advanced codecs. A rate-distortion optimal waymay be understood such that, for example, a maximum bit rate for a bitstream is specified and an inventive encoder is operative to choose thecolor-space-representation of the residual signal that provides the bestencoding quality at the specified bit rate. However, it is also possibleto optimize the rate for a fixed quality or optimize a R/D ratio by useof some cost function.

The quality-bit rate dependency is, for a single sample frame, plottedin FIG. 2.

As can be seen, a specified maximum bit rate is given on the x-axis (inunits of Mbits/Sec) and the corresponding image quality (signal-to-noiseratio in units of dB) is plotted on the y-axis. FIG. 2 shows two socalled “rate-distortion performance curves” for encoding a singlepicture in two different fixed color-space-representations. The firstcurve 120 shows the rate dependency of the picture when RGB is chosen ascolor-space-representation and the second curve 122 shows therate-distortion performance when YCoCg is chosen ascolor-space-representation. FIG. 2 shows the known effect that a singlecolor-space-representation cannot be optimal (in a rate-distortionsense) for all different source picture characteristics. In general, theamount of correlation between the R, G and B channels is highly signaldependent and may even change within a given picture.

FIG. 2 shows the rate-distortion (R-D) curves for a typical intra-onlycoding scenario, where the color-space-representations have been fixedbefore encoding. Curve 120 represents the R-D performance obtained forthe case of encoding in the original RGB domain, while encoding of thesame source in the YCoCg color-space results in an R-D performance shownby FIG. 122. It may be noted that the distortion (D) in the plot hasbeen measured as the average of the R, G, and B picture peaksignal-to-noise ratio values, that is by comparing the original picturewith the additional noise introduced by the encoding.

It may be noted that the curves in FIG. 2 represent averaged data for acomplete picture. The effects discussed in the following paragraphs withrespect to bit rate may be much more dominant when observed for singlemacro-blocks since averaging effects then do not occur and thedifference of the quality achieved by using differentcolor-space-representations on the single macro-block level may even bebigger.

As can be seen from the R-D curves 120 and 122 in FIG. 2, low bit rateencoding using the YCoCg representation performs significantly betterthan that using the corresponding RGB representation. On the other hand,RGB-based encoding leads to an increasingly better performance whenmoving towards higher bit rates with more and more noise componentsgetting encoded. As a consequence, there is a crossover region 123indicating a sub-optimal R-D performance of both alternativerepresentations since in either case, for encoding the sample in asingle color-space-representation one can only move along one or theother R-D curve. Using an inventive decoder 100 and a correspondinginventive encoder, the present inventive concept solves this problem andis achieving a coding performance corresponding to curve 124, which isthe R-D envelope of both the RGB-based and YCoCg-based R-D curves.

Moreover, in many coding applications neither the specific codingconditions nor the typical characteristics of the source are knownbeforehand. Using the inventive decoder and corresponding inventiveencoders, the optimum color-space-representation can be adaptivelychosen to be optimum in a rate-distortion sense.

FIG. 3 gives an example for a conversion of a nearly gray signal fromthe RGB-color-space to the YCoCg-color-space to further explain theinventive concept and the mechanisms leading to a potential decrease inbit rate. The color transform from the RGB to the YCoCgcolor-space-representation can be performed in a reversible way byapplying the following operations to each triple of (R, G, B) or(Y,Co,Cg) values, respectively:

Co=R−B  t=Y−(Cg>>1)

t=B+(Co>>1)  G=Cg+t

Cg=G−t

B=t−(Co>>1)

Y=t+(Cg>>1)  R=B+Co.

In the above notation, the operator (>>) means bitwise movement of theunderlying bit-string to the right and is thus equivalent to a divisionby 2.

It may again be noted that the inventive idea does not depend on theexact choice of the color-space-representations to switch between. Inthe given examples, the restriction to the citedcolor-space-representations is mainly because of the fact that they arewidely used.

FIG. 3 shows a graphical representation of a color-space transformationfrom the RGB color-space to the YCoCg color-space. The originalRGB-signal 140 exemplarily has nearly equally valued R, G and Bparameters, i.e. the corresponding pixel is nearly gray with anintensity proportional or depending on the sum of the RGB values. Sincethe pixel in question is nearly colorless, a transformation to the YCoCgcolor-space does provide parameter values that are close to zero for thechroma parameters Co and Cg, resembling the fact that the signal isnearly colorless. On the other hand, the luma parameter Y is having arather big value compared to the chroma parameters.

The example shown in FIG. 3 shows a content that is predominantly lesscolor saturated in which the usage of a decorrelating color transformfrom RGB to, for example, YCoCg may be very helpful in terms of overallcoding efficiency since in that case the corresponding tristimulusvalues (values of the single information channels within onecolor-space-representation) are closer to being equal to one another. Ifwithin one picture, the color saturation is rather low, the individualRGB values might differ to some extend. The sum, i.e. the Y-parameter ofthe YCoCg-representation may then be varying smoothly over the image,and, due to the low color saturation, the Co and Cg parameters arerather small. Such smoothly or nearly constant parameters can be encodedmore efficiently.

Such, the effectiveness of a color transform may be highly dependent onthe specific coding conditions. This is especially true for sources thatcontain a high amount of signal-independent, uncorrelated noise in theprimary channels. The color transform from RGB to YCoCg, when written ina matrix form, has matrix elements off the diagonal that are rathersignificant in value. The “amplification” of the Y-channel above aquantization threshold 152, which is shown for illustrative purposesonly, is directly connected to these off-diagonal elements. Therefore,for the sources containing a high amount of signal-independent,uncorrelated noise, the significant off-diagonal elements of adecorrelating color transform may cause a severe amplification of thenoise, which in turn results in a degradation of coding efficiency inthe high bit-rate range where the noise components typically aresupposed to survive the quantization process.

As mentioned before, with respect to FIGS. 2 and 3, it can be extremelybeneficial to adapt the color representation to the characteristics ofthe given prediction residual signal on a macro-block by macro-block(picture-block by picture-block) basis. Therefore, within a bit streamcomprising the prediction residual signals, a new syntax element couldbe introduced in the bit stream. That syntax element could for example,when being equal to one, indicate encoding and decoding of the givenmacro-block involving the application of the color-space transformationby invoking the corresponding forward and inverse transform operationsshown before. That introduced flag could, when being equal to zero ornot present, further mean that the encoding and decoding processproceeds in the same way as already specified before, i.e. based on theoriginal color space that existed before encoding.

FIG. 4 shows an inventive encoder 200 for generating a representation ofa picture having multiple picture blocks that are carrying pictureinformation for picture areas that are smaller than the area of the fullpicture 200. The encoder 200 has a color-space transformer 202 fortransforming the picture information of picture blocks from a firstcolor-space representation (A) to a second color-space representation(B).

Encoding the picture on a picture-block basis, the individualpicture-blocks 210A to 210F are input into the inventive encoder 200.The encoder outputs encoded picture blocks either in a first color-spacerepresentation (A) or in a second color-space representation (B).

The encoder 200 may further comprise a transformation decider 214 thatdecides on a picture-block by picture-block basis, whether thetransformation for the processed picture-block shall be performed. Thetransformation decider 214 can, for example, meet the transformationdecision based on a maximum alllowable bit rate, choosing thecolor-space representation providing the best possible quality at thegiven bit rate.

Another possibility would be to define a desired maximum quality(closely connected to the coarseness of quantization), i.e. a desireddistortion value, and the transformation decider 214 is working on a tryand error basis, where the individual picture-blocks are generallyencoded in both color-space representations and the transformationdecider 214 is choosing the color-space transformation resulting in thelower bit rate. Of course, every other decision rule may be used by thetransformation decider, for example, based on analytical expressions orestimations based on previously tabulated sample configurations. Theinventive encoder 200 may furthermore be operative to incorporatetransformation information indicating a desired transformation for agiven picture block to the bit stream also having the information on thepicture blocks. This signals to a corresponding decoder, whether acolor-space transformation is to be performed on the decoder side ornot.

When introducing such an additional flag as proposed before to signalwhether the color-space transformation is to be performed for amacro-block in question or not, further bit rate can be saved by entropyencoding this introduced flag, for example called mb_rct_flag(“macroblock residual color transform flag”). To achieve an efficientcoding, an arithmetic coding concept can, for example, be applied tocode the binary data. Therefore, the chosen arithmetic coding could be abinary arithmetic coding concept, relying on the probability ofoccurrence of values 0 or 1 per bit (or per mb_rct_flag concerning aspecific macro-block). Furthermore, it would, for example, beadvantageous to implement the binary arithmetic coding in an adaptivemanner, i.e. in a way that the underlying probability distribution ofthe arithmetic coding algorithm is “learning” or updated in dependenceon the actual occurance of mb_rct_flag's already having been encoded.That is, that the probabilities of the occurrence of the single bitvalues are updated once a real value is observed and thus the underlyingprobability distribution is adapted to the actual.

Furthermore, the adaptive binary arithmetic coding can also beimplemented in a context sensitive manner, i.e. different probabilitydistributions are at hand for different defined contexts. In otherwords, more than one context could be spent for mb_rct_flag. One exampleof a context description is shown in FIG. 4A where, within a picture240, three macro-blocks 242 a, 242 b and 242 c are shown. If, forexample, macro-block 242 a is to be encoded, the context, i.e. theenvironment condition of the macro-block to be coded, could be derivedby the neighboring left (a) macro-block 242 b and by the neighboringupper (b) macro-block 242 c. Based on the mb_rct_flag's of thesemacro-blocks, 3 different contexts ct×Id×Inc can be berived by thefollowing expression:

ct×Id×Inc(C)=(mb _(—) rct_flag (A)==0): 0 ? 1+(mb _(—) rct_flag (B)==0)? 0:1.

According to an alternative notation, this could be written as:

ct×Id×Inc(C)=mb _(—) rct_flag (A)+mb _(—) rct_flag (B).

It should be noted that, as already mentioned above, the mb_rct_flags donot necessarily have to be present for each individual macro-block. Itis to be supposed that the flag is equal to 0 when not present for theevaluation of the above formula.

One may, for example, further foresee an additional functionality, whichis also signaled by a flag “rct_mode_flag”. This flag can switch thecolor-space-transformation on and off for a greater sample ofmacro-blocks that are forming, for example, a slice of macro-blocks thatshares together some other distinct properties. Only if rct_mode_flag isequal to 1, mb_rct_flag's shall be present in the macro-block layer.

FIG. 5 illustrates, for a simplified example, the encoding process usingmotion estimation and predictive residual coding. The encoding shall beshortly explained on a basis of two consecutive pictures 250 and 252.Motion estimation is presented with the help of a sample macro-block254A in picture 252.

The picture content of the macro-block 254A is also found during amotion estimation step in the picture 252, called reference picture. Inthe reference picture the corresponding macro-block 254B is displaced bya motion vector 256 from its position 254A in picture 252. In case themacro-block 254B has not changed its content at all, a straightforwardway for deriving the picture portion of picture 252 that corresponds tothe position of the macro-block 254B would be to simply transmit themotion vector 256 within a bit stream. This enables a decoder toreconstruct picture-block 254B at the appropriate position, when thedecoder has knowledge of the preceding picture 250.

In a more general scenario, the picture content of the macro-block 254Bwill have changed with respect to the picture content of thecorresponding area 254A in the reference picture 250. In predictivecoding, only the difference of the prediction 254A to the actual content254B is transmitted, since the residual samples are expected to be smalland therefore can be coded using low bit rate. Thus, in addition to themacro-block 250A and the motion vector 256 the residual signal 258 hasto be computed and used for a representation of the finally transmittedsignal. According to the present invention, the finally transmittedsignal can either be transmitted in a first color-space representation258A or in a second color-space representation 258B depending on the bitrate or bandwidth of a transmission channel available.

It is noted here that having a single motion vector for all three signalcomponents (e.g. R, G and B), i.e. the reference information is derivedfrom the same block of the same reference picture, is the simplestpossible case. In a more general approach, different motion vectors foreach signal component can be derived, i.e. the reference information isderived from different picture blocks, that can additionally originatefrom different reference pictures. The present invention is thus notnecessarily restricted to the case of having one motion vector, i.e.,the same prediction operator for all three components. It is for examplean advantageous embodiment of the present invention to have one singlemotion vector.

As already mentioned above, the application in a macro-block basedcoding scheme using predictive residual coding is an advantageousapplication scenario, since then the required bit rate canadvantageously be further decreased by simple and computationally cheapcolor-space transformations.

FIG. 6 shows an inventive bit stream 300 having multiple bit streamrepresentations of picture-blocks 302A to 302C that can be provided in afirst color-space representation (A) or in a second color-spacerepresentation (B). The inventive bit stream can be used by an inventivedecoder allowing for a highly compressed transmission of a compressedpicture or a compressed picture sequence by a transmission channel, thatmay be wired, wireless, or the like. Of course the storage of aninventive bit stream on a computer-readable storage medium is alsopossible, having the advantage of requiring only little storage space.The bit stream may further comprise indication information 304indicating the desired color-space-transformation of picture-block 302B.

Although the previously described embodiments of the present inventionhave been described mainly using the RGB and YCoCg-spaces, the presentinvention is not at all limited to the use of those color-spaces. In afurther embodiment, arbitrary other color-spaces or other means ofdecorrelating inter-color techniques may be used and it is even possibleto provide an inventive encoder or decoder capable of transformingbetween three or more different color-space representations.

Although the present invention has been mainly described with respect tovideo coding, it may also advantageously be used for coding of stillimages. Furthermore, the number of samples may be varied.

Depending on certain implementation requirements of the inventivemethods, the inventive methods can be implemented in hardware or insoftware. The implementation can be performed using a digital storagemedium, in particular a disk, DVD or a CD having electronically readablecontrol signals stored thereon, which cooperate with a programmablecomputer system such that the inventive methods are performed.Generally, the present invention is, therefore, a computer programproduct with a program code stored on a machine-readable carrier, theprogram code being operative for performing the inventive methods whenthe computer program product runs on a computer. In other words, theinventive methods are, therefore, a computer program having a programcode for performing at least one of the inventive methods when thecomputer program runs on a computer.

While the foregoing has been particularly shown and described withreference to particular embodiments thereof, it will be understood bythose skilled in the art that various other changes in the form anddetails may be made without departing from the spirit and scope thereof.It is to be understood that various changes may be made in adapting todifferent embodiments without departing from the broader conceptsdisclosed herein and comprehended by the claims that follow.

1-25. (canceled)
 26. A decoder for reconstructing a picture representedin an entropy encoded representation having a first and a second pictureblock, the picture blocks carrying picture information for picture areassmaller than the area of the picture, wherein the first picture block iscarrying the picture information in a first color-space representationand the second picture block is carrying the picture information in asecond color-space representation, the decoder comprising: an entropydecoder for deriving the first and second picture blocks using anentropy decoding rule; and a color-space transformer for transformingeither the color-space representation of the first picture block to thesecond color-space representation or the color-space representation ofthe second picture block to the first color-space representation. 27.The decoder in accordance with claim 26, in which the color-spacetransformer is further operative to process transformation indicationinformation indicating a desired transformation for a picture block; andin which the decoder is further having a flag receiver for receiving thetransformation indication information.
 28. The decoder in accordancewith claim 26, in which the color-space transformer is operative toprocess the RGB-color-space and a second color-space representationcomprising one luma-parameter indicating a brightness and two chromaparameters indicating a chromatic composition of a signal.
 29. Thedecoder in accordance with claim 28, in which the color-spacetransformer is operative to perform the color-space transformationbetween the RGB-color-space described by parameters R, G, and B and thesecond color-space representation described by the luma parameter Y andthe chroma parameters Cg and Co according to the following formulas:Co=R−B  t=Y−(Cg>>1)t=B+(Co>>1)  G=Cg+tCg=G−t

B=t−(Co>>1)Y=t+(Cg>>1)  R=B+Co.
 30. The decoder in accordance with claim 26, inwhich the color-space transformer is operative to perform thetransformation based on a parametric representation of the pictureinformation within the picture blocks, the parametric representationdescribing the picture information in a transform domain.
 31. Thedecoder in accordance with claim 30, in which the color-spacetransformer is operative to perform the transformation based on aparameter representation describing the picture information in afrequency domain.
 32. The decoder in accordance with claim 26, in whichthe entropy decoder further comprises a requantizer for deriving t thefirst and the second picture block from a quantized representation ofentropy decoded picture information.
 33. The decoder in accordance withclaim 26, in which the entropy decoder is operative to use an entropydecoding rule comprising the use of a Variable-length-codebook.
 34. Thedecoder in accordance with claim 26, in which the entropy decoder isoperative to use an entropy decoding rule comprising the use of a binaryarithmetic coding algorithm.
 35. The decoder in accordance with claim26, in which the entropy decoder is operative to use a decoding rulehaving one or more sub-rules chosen depending on a decoding context. 36.The decoder in accordance with claim 26, in which the decoder isoperative to reconstruct the picture using information from referencepictures of a picture stream, which are temporarily preceding orfollowing the picture within the picture stream and that are representedusing related picture blocks corresponding to the picture blocks of thepicture, the related picture blocks having picture information on thesame picture content as the picture blocks, wherein a positional changebetween the picture blocks and the corresponding picture blocks of thereference pictures with respect to a fixed location of the given pictureblocks can be described by motion vectors.
 37. The decoder in accordancewith claim 36, in which the decoder is operative to reconstruct thepicture blocks using the corresponding picture blocks and differentialpicture blocks predicting a change in picture information of the pictureblocks with respect to the corresponding picture blocks.
 38. The decoderin accordance with claim 36, further comprising an input interface forreceiving a bit stream representation of the picture stream having theinformation of the single pictures of the picture stream.
 39. Thedecoder in accordance with claim 26, further having a picture composerfor reconstructing the picture using the first and the second pictureblock.
 40. The decoder in accordance with claim 26, in which thecolor-space transformer is further operative to process bypassinformation indicating a sequence of picture blocks and to switch offcolor-space transformation for the sequence of picture blocks indicatedby the bypass information.
 41. An encoder for generating arepresentation of a picture having a first and a second picture block,the picture blocks carrying picture information for picture areassmaller than the area of the picture in a first color-spacerepresentation, the encoder comprising: a color-space transformer forderiving transformed picture blocks, the color-space transformer adaptedto transform the picture information of one of the first and the secondpicture blocks to a second color-space representation; and an entropyencoder for deriving an entropy encoded representation of thetransformed picture blocks according to an entropy encoding rule. 42.The encoder in accordance with claim 41, in which the color-spacetransformer is operative to transform the first or the second pictureblock for optimizing a rate-distortion ratio of the entropy encodedrepresentations of the transformed picture blocks using a cost function.43. The encoder in accordance with claim 41, in which the color-spacetransformer is operative to process a transformation informationindicating a picture block to be transformed; and further comprising atransformation decider for generating the transformation informationusing a decision rule.
 44. The encoder in accordance with claim 43, inwhich the transformation decider is operative to use a decision rulethat is selecting the picture block requiring less information unitswhen transformed to the second color-space representation.
 45. Theencoder in accordance with claim 44, in which the transformation decideris operative to use a decision rule that is selecting the picture blockin a rate-distortion optimal way.
 46. The encoder in accordance withclaim 42, further comprising an output interface for outputting a bitstream having the picture information including the entropy encodedrepresentations of the transformed first and the second picture blocks.47. The encoder in accordance with claim 41, further comprising a motionestimator adapted to derive motion vectors indicating a positionalchange between related picture blocks and the picture blocks, therelated picture blocks being contained within reference pictures of apicture stream and having picture information on the same picturecontent as the picture blocks, wherein the reference picturestemporarily precede or follow the picture within the picture stream. 48.A method of decoding a picture represented in an entropy encodedrepresentation having a first and a second picture block, the pictureblocks carrying picture information for picture areas smaller than thearea of the picture, wherein the first picture block is carrying thepicture information in a first color-space representation and the secondpicture block is carrying the picture information in a secondcolor-space representation, the method comprising: entropy decoding theentropy encoded representation for deriving the first and second pictureblocks using an entropy decoding rule; and transformation of either thecolor-space representation of the first picture block to the secondcolor-space representation or the color-space representation of thesecond picture block to the first color-space representation.
 49. Amethod of generating a representation of a picture having a first and asecond picture block, the picture blocks carrying picture informationfor picture areas smaller than the area of the picture in a firstcolor-space representation, the method comprising: transformation of thepicture information of the first or of the second picture block to asecond color-space representation to derive transformed picture blocks;and entropy encoding the transformed picture blocks for deriving anentropy encoded representation of the transformed picture blocksaccording to an entropy encoding rule.
 50. A parameter bit stream havingan entropy encoded representation of a picture having a first pictureblock and a second picture block, the picture blocks carrying pictureinformation for picture areas smaller than the area of the picture,wherein the first picture block is carrying the picture information in afirst color-space representation and the second picture block iscarrying the picture information in a second color-space representation.51. A computer readable medium storing a computer program forperforming, when run on a computer, the method of decoding a picturerepresented in an entropy encoded representation having a first and asecond picture block, the picture blocks carrying picture informationfor picture areas smaller than the area of the picture, wherein thefirst picture block is carrying the picture information in a firstcolor-space representation and the second picture block is carrying thepicture information in a second color-space representation, the methodcomprising: entropy decoding the entropy encoded representation forderiving the first and second picture blocks using an entropy decodingrule; and transformation of either the color-space representation of thefirst picture block to the second color-space representation or thecolor-space representation of the second picture block to the firstcolor-space representation.
 52. A computer readable medium storing acomputer program for performing, when run on a computer, the method ofgenerating a representation of a picture having a first and a secondpicture block, the picture blocks carrying picture information forpicture areas smaller than the area of the picture in a firstcolor-space representation, the method comprising: transformation of thepicture information of the first or of the second picture block to asecond color-space representation to derive transformed picture blocks;and entropy encoding the transformed picture blocks for deriving anentropy encoded representation of the transformed picture blocksaccording to an entropy encoding rule.